Wellbores are formed in subterranean formations for various purposes including, for example, the extraction of oil and gas and the extraction of geothermal heat. Such wellbores are typically formed using one or more drill bits, such as fixed-cutter bits (sometimes referred to in the art as polycrystalline diamond compact or PDC bits), rolling-cutter bits (sometimes referred to in the art as “rock” bits), diamond-impregnated bits, and hybrid bits, which may include, for example, both fixed cutters and rolling cutters. The drill bit is coupled either directly or indirectly to an end of a drill string or work string, which encompasses a series of elongated tubular segments connected end-to-end that extends into the wellbore from the surface. Drilling is a process of forming the wellbore by rotating the drill bit so that its cutters or abrasive structures cut, crush, shear, and/or abrade away the formation materials.
To enlarge the diameter of the wellbore, a “reamer” (also referred to as a “hole opening device” or a “hole opener”) may be used in conjunction with the drill bit as part of a bottom-hole assembly (BHA). The reamer is typically axially-offset and uphole from the drill bit along the length of the BHA. In operation, the drill bit operates as a pilot bit to form a pilot bore in the subterranean formation, and the reamer follows the drill bit through the pilot bore to enlarge the diameter of the wellbore as the BHA advances into the formation.
Various non-ideal drill string behaviors can occur while drilling due to the complex dynamic behavior of the drill string and its interaction with the formation being drilled. One such mode of undesirable drill string behavior is known as stick-slip. During drilling, the drill string can be elastically twisted (i.e. torsionally flexed without appreciable yielding), up to several full 360-degree revolutions, while the drill bit temporarily sticks due to friction between the drill bit and the formation. Torsion in the drill string builds to an excessive value that eventually frees the drill bit, causing the freed drill bit to rotate violently with an angular velocity that is temporarily much higher than the angular velocity measured at the surface. Stick-slip causes excessive and unwanted vibrations for a drill string in the torsional direction, along with excessive drill bit speeds, which can lead to premature bit wear or failure of the drill bit or other drill string components.
Another mode of undesirable drill string behavior is known as “bit whirl.” During drilling, the intended rotational motion of the drill string is around its own central axis. Bit whirl is an additional bulk rotation of the drill string, which is eccentric or precessing rotation of the drill string offset from the wellbore axis. This additional bulk rotation can be induced due to bending forces on the drill string in combination with the spinning rotation of the drill string about its own axis. Whirling motion can occur in the same direction as the rotation of the drill string (forward whirl) or in the opposite direction (backward whirl). Backward whirl is known to be a particularly strong cause of PDC (polycrystalline diamond compact) drill bit failures and of lower performance of PDC drill bits.
Efforts to reduce or eliminate unwanted drill string behavior such as stick-slip and bit whirl include modeling drill string behavior to identify causes and solutions.